TL;DR: more operation cost then it seems, limited AD income, many streamer on which they lose money, increasingly more competition with "unfair advantage" (second platform effect, YT/TickTock; unethical content Kik(gambling); unethical monetization strategies TickTock).
operating a reasonable reliable live streaming service is quite expensive and Twitch is not Amazone. Sure it's owned by it but it still has to buy all computation resources from Amazone, for similar prices then some of the competition like Kick (which deeply interwinded with gambling streams which are profitable but ethically and legally problematic and hence banned on Twitch)
and while they do take a cut this is only profitable if a streamer gets enough subs/bits etc.
but most streamers on twitch do not get that, but might still have people watching across two continents+. E.g. having less then 50 viewers but viewers across 2 continents with a stream earning less 1€ income (not profit) per-hour in average isn't that rare
So the "big streamers" would need to subvention the cost of Twitch being free for streamers, which isn't easy. That some people do use Twitch as a form of marketing platform to goat people on other platforms and then there earn money from then doesn't help (and given how little twitch does against that but often simply could I'm seriously wondering if there is some internal employee corruption scandal to be uncovered).
Then there is another issue Twitch has compared to YT, it ADs are worth way less. Actually often Twitch doesn't even get enough ADs bought out so that some people in some regions sometimes see no or hardly any ADs (no one bought any) or ADs repeat too often...
Lastly and maybe for some people surprisingly given how some tech channels love to blow up any small negative news about twitch it's trying to act "reasonable ethically", i.e. not "oh they are grate people ethically" but "they have limits on how evil they act which are more then the law requires" ethically. You can see this if you compare the usage of dark patterns and highly addictive feedback loops between Twitch and TickTock. The later maxes out everything they can wrt. dark patterns which have addictive effects (at least for vulnerable people) and obfuscate how much money you spend. While Twitch, well does not do so.
Oh and probably there was some mismanagement, not now but a few years ago.